'07 Iron Horse LE tire question

I have my Iron Horse in my van but forgot to look at the tire size before I left for work this mourning, didn't drive the van. Need a rear tire and went to order it on Jenson Usa and the sizes are 26x2.1 or 26x2.3 WTB Weirwolf. Any help on what my OE one is, can I go with the 2.3 on the rear? any reason I should go with the narrower one for the rear? I think stock the front was a little wider and the rear was slightly smaller? advantage to that?